(1) Field of the Invention
The present invention relates to a method of manufacturing hollow rods. The term "hollow rods" herein generally refers to extra-thick-walled hollow rods of the type which are produced at steel rod rolling mills at large, and more particularly to such rods having a wall thickness to diameter ratio (hereinafter referred to as T/D) of 25% or above which cannot be manufactured into seamless tubes at any existing mandrel mill plant, typical of such rods being those for manufacture of oil-well drill collars.
(2) Description of the Prior Art
The manufacture of hollow rods of the type is conventionally carried out in manner as illustrated in FIG. 1, a schematic view showing a conventional process of manufacturing hollow rods. That is, a billet B.sub.1 is passed through a rolling mill 31 into a square billet B.sub.2 having specified dimensions (FIG. 1(a)); the square billet B.sub.2 is centrally pierced into a square hollow billet B.sub.3 by using a drill 32 (FIG. 1(b)); then a mandrel 33 of manganese steel is inserted into the hollow billet B.sub.3 (FIG. 1 (c)); the hollow billet B.sub.3 having the mandrel 33 so inserted is heated to the specified temperature in a heating furnace 34 (FIG. 1(d)); thereafter, it is passed through a bar mill 35 consisting of more than ten roll stands, each having caliber rolls 35a, so that the hollow billet B.sub.3 is finished to the desired diameter and wall thickness (FIG. 1(e)); the mandrel 33 is removed from the hollow billet B.sub.3 and the latter is cut to the specified length (FIG. 1(f)); and subsequently, the hollow billet B.sub.3 is subjected to bend straightening by a straightening machine 36 into a hollow rod B.sub.4 as a product (FIG. 1(g)).
The conventional manufacturing process as above described involves the following problems: (1) the hollow billet B.sub.3 is rolled, with the mandrel 33, an internal sizing tool, inserted therein, and since the mandrel 33 is subject to plastic deformation, the product is unsatisfactory in roundness and liable to wall eccentricity; (2) the product is considerably inconsistent in inner diameter, which means inconsistency of the wall thickness, and accordingly its dimensional accuracy is low as a whole; and (3) the mandrel 33, which is subject to plastic deformation, is to be discarded after use, and accordingly the unit tool requirement is costly and uneconomical. With a view to overcoming one of these difficulties, or the disadvantage that the unit tool requirement is costly due to the use of a mandrel, it has been proposed to produce hollow rods in the following manner. That is, a bloom is pierced into a hollow piece by employing a press piercing mill, and then the hollow piece is reduced by being passed through a continuous rolling machine having caliber rolls of oval round type arranged in an alternately horizontal-vertical pattern, without using any internal sizing tool (Japanese Patent Kokai No. 114,407 of Showa 55).
However, experiments by the present inventors have proved that with such method it is extremely difficult to obtain sufficient roundness, where two-high mill type roll stands having caliber rolls are employed. It has also been confirmed that where block mill of three-roll type having caliber rolls are employed, better roundness can be obtained than where two-mill type is employed, but that such method has still its limitation.
Beside said method there is known one similar to the method of the present invention, that is, U.S. Pat. No. 374,703 entitled "Rolls for reducing and tapering tubes and rods" (Dec. 13, 1887). However, these rolls are different from those according to the invention, in configuration and the relationship between cross angle and the position of each larger diameter side roll portion. And there is known one similar to the method of the present invention, this is U.S. Pat. No. 3,495,429 entitled "Method of reducing tubes, especially thick-walled tubes and means for practicing the method" (Feb. 17, 1970). However, this method is different from the present invention, in configuration of the rolls and the inside diameter of a hollow rod is not reduced according to the present invention.
There is known another similar method claimed by one of the present inventors and entitled "Process for manufacturing seamless metal tubes (Fil. No. 06/281,901). This prior method consists essentially in subjecting a shell being worked to outside-diameter reduction by means of a rotary mill having three or four rolls arranged around a pass line and without using internal sizing tools, the axes of the rolls being inclined or inclinable so that the shaft ends on either side of the rolls stay close to or stay away from the pass line, said axes being inclined so that the shaft ends on the respective sides of the rolls face in the peripheral direction on one and the same side of the shell being worked.
In other words, the principle of said method is based on the fact that where the shell has some wall eccentricity if its outside diameter is reduced so that the wall thickness is increased, the degree of thickness increase is greater in a thin wall portion than in a thick wall portion, wall thickness of the shell being equalized in the light of such fact. According to experiments by the present inventors, however, such wall equalization is achievable only where T/D is 25% or lower, and indeed it has been confirmed that where T/D is more than 25%, it is physically impossible to increase the wall thickness by outside-diameter reduction according to the prior method. This means that the method of said prior application is intended for use only where T/D is 25% or below. On the other hand, the method of the present invention is applicable in the case where T/D is 25% or above. Furthermore, whereas the prior method is one for diameter reduction in which wall thickness is increased, the present invention is intended to effect elongation so that the wall thickness, as well as the outside diameter, is reduced. As such, it is obvious that the two methods are entirely different in subject matter.